1. Field of the Invention
One disclosed aspect of the embodiments relates to a light-emitting element array including a plurality of surface light-emitting elements in an array.
2. Description of the Related Art
A vertical cavity surface emitting laser (hereinafter referred to as VCSEL) is a laser that emits light in a direction perpendicular to the in-plane direction of the semiconductor substrate. The reflector used in a VCSEL is typically a distributed Bragg reflector (hereinafter referred to as DBR). The DBR generally has a multilayer structure including low-refractive-index layers and high-refractive-index layers alternately formed to an optical thickness of λ/4 each. λ used herein represents the wavelength of light emitted from a light-emitting element. The optical thickness of a layer is represented by the product of the thickness of the layer and the refractive index of the material of the layer.
For a VCSEL using nitride semiconductor materials, it is difficult to increase the difference in refractive index between the materials, and accordingly, reflection characteristics of the semiconductor DBR are limited. Also, the materials of the layers of the DBR have a large difference in lattice constant therebetween, and crystal growth of a semiconductor DBR is difficult. In generally used VCSELs, accordingly, the lower DBR (closer to the substrate) is a semiconductor DBR, whereas the upper DBR is a dielectric DBR.
In general, the output power of semiconductor lasers is limited due to a temperature increase caused by heat generated from working elements. One of the common measures to suppress such a temperature increase is to increase the heat radiation performance of the laser.
In a VCSEL using nitride semiconductors, unfortunately, a dielectric DBR having a low thermal conductivity lies at the upper side of the element. This makes it difficult to enhance heat radiation performance. In addition, if the semiconductor DBR at the lower side is made of binary alloy semiconductor materials, such as AlN and GaN, the thermal conductivity of the semiconductor DBR can be increased, whereas the materials have a large difference in lattice constant therebetween. Such a semiconductor DBR is difficult to make. On the other hand, if the semiconductor DBR is made of ternary alloy semiconductor materials, such as AlGaN and AlInN, the production process thereof can be easier than the process using binary alloy semiconductor materials. However, the thermal conductivity of such a semiconductor DBR is low, and heat dissipation therefore decreases.
Japanese Patent Laid-Open No. 11-150300 discloses the technique of enhancing heat radiation performance by dissipating heat through a cathode disposed on an n-type nitride semiconductor layer.
High power nitride semiconductor lasers have recently been used as, for example, excitation light sources of titanium-sapphire (Ti:Sa) lasers, which are a type of solid-state laser. In this use, the excitation light source of the solid-state laser requires a high-power light source. Accordingly, the use of a VCSEL array including nitride semiconductor VCSELs in an array is effective. The optical output power of a VCSEL array is determined by multiplication of the optical output power of one VCSEL element of the VCSEL array and the number of VCSEL elements. By closely integrating VCSEL elements into a VCSEL array, high output power can be achieved.
However, if VCSEL elements are closely integrated into an array, the output power of each VCSEL element is reduced due to thermal cross-talk between the VCSEL elements, and the output power per unit area of the VCSEL array is reduced accordingly.
In the case of using a single element as in the case of the above-cited patent document, heat radiation performance can be enhanced by increasing the area of the electrode. In the case of the VCSEL array including a plurality of VCSEL elements, however, the increase of the area of the electrode leads to reduced integration in the VCSEL array and results in reduced optical power per unit area of the VCSEL array.
In order to increase the optical power per unit area of a VCSEL array produced by integrating VCSEL elements, both the heat radiation performance of each VCSEL element and the integration degree of the array must be improved.